[debugger] remove step count parameter from prepare step.

test/mjsunit/regress/regress-crbug-409614.js

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Flags: --expose-debug-as debug
 
 Debug = debug.Debug;
 var exception = null;
 var error_count = 0;
 
 function f() {
   return 0;  // Break
-}
+}            // Break
 
 function listener(event, exec_state, event_data, data) {
   if (event != Debug.DebugEvent.Break) return;
   try {
     if (exec_state.frame(0).sourceLineText().indexOf("Break") <0) {
       error_count++;
     }
-    exec_state.prepareStep(Debug.StepAction.StepIn, 2);
+    exec_state.prepareStep(Debug.StepAction.StepIn);
     f();  // We should not break in this call of f().
   } catch (e) {
     print(e + e.stack);
     exception = e;
   }
 }
 
 Debug.setListener(listener);
 
 debugger;  // Break
-f();
+f();       // Break
 
 Debug.setListener(null);  // Break
 
 assertNull(exception);
 assertEquals(0, error_count);